Production and recovery of para-xylene



UNITED STATES PATENT OFFICE PRODUCTION AND RECOVERY OF PABA-XYLENE Stanley Francis Birch, Frederick Arnold Fidler,

and John Norman Haresnape, Snnbnry-ongilames, England, assignors to Anglo-Iranian Company Limited, London, England, a

British joint-stock corporation No Drawing. Application April 3, 1947, Serial No. 789,281. In Great Britain April 12, 1946 1 ii Claims.

The invention relates to the production and recovery of para-:wlene, and is more particularly concerned in obtaining an increased yield of para-xylene from hydrocarbon mixtures in which it is contained. 5 to conduct the distillation as a continuous proc- Hitherto para-xylene was obtained by separaess. it will be understood that the separation of tion in known manner from petroleum and coal these fractions may also be carried out in a suittar xylene fractions, normally consisting essenable batch distillation unit. tially of ortho-, metaand para-xylenes and The para-xylene concentrate is cooled to a low ethyl benzene, the content of para-xylene vary- 1o temperature of he Order 01 0 to ry llis ing between and out the para-xylene and the residue, together We have now found that it is possible to isomwith the metaand ortho-xylenes passed to the erise the orthoand meta-xylenes contained isomerisation stage. The product from the isomin such xylene fractions to para-xylene under erisation stage is passed to a fractionating colcarefully controlled conditions, and thereby to in- 15 umn as used for the first fractionation, to procrease the amount of para-xylene recoverable im 8- lven-Xyl ne oncentrate and a r sidue from such fractions. consisting largely of metaand ortho-xylenes, Accordingtothe invention, an aromatic hydrothe para-xylene being added to the previously carbon fraction of coal tar or petroleum origin p r 'p r yl n th r i e of rth and boiling in the range of 135-150 c. is treated 2 and metayl n s b n tu n d to the isom rin known manner to separate the ethyl benzene isation stage. and para-:wlene from the orthoand meta- According to a further modification of the xylenes. which are then passed to a catalytic isomprocess of the invention the aromatic xylene fracerisation stage in which part of the metaand tion is first fractionated to remove the ethyl benorthoxylenes are converted into para-xylene, ene. the residue consisting essentia y the and thepara-xylene recovered from the isomermeric xylenes then being cooled to about -40 C. isation product. a to 50 0., to remove the bulk of the para-xylene In a preferred method of carrying out the inby ry llis ion, th m h r liq containing vention an aromatic hydrocarbon fraction boilh r h and m -xyl n b in passed to a ing in the range l35-l50 C., is'fra'ctionated to catalytic isomerisation stage in which part of the produce a light fraction boiling up to about 137 orthoand meta-xylenes are converted into parac.. and containing the bulk of the ethyl benzene xylene. and the para-x l n s parat d fr m the present in the aromatic hydrocarbon fraction, lsomelisatlon p and an intermediate fraction of narrow boiling According to a still further modification of the range and containing a high proportion of paraprocess of the invention the aromatic xylene fracxyiene. and a residue consisting essentially of tion is first fractionated to provide an overhead orthoand meta-xylenes. The intermediate fracfraction rich in ethyl benzene, para-xylene and tion is cooled to a temperature within the range -Xy ene. an intermediate out co t a 0-50 C. in order to remove the bulk of the parahigh concentration of meta-xylene, and a residue xylene by crystallisation, the mother liquors to- 0 containing ortho-xylene and higher boiling aroether with the residue from the fractionation matics. The overhead fraction is cooled to about stage being passed to a catalytic isomerisation -40 C. to C. and centrifuged whereby a stage in which part of the orthoand meta-xysolid of high concentration of para-xylene is seplene are converted into para-xylene, and the arated and the mother liquors thereafter fracpara-xylene separated from the isomerisation 45 tionally distilled in a column of about theproduct. oretical plates. In this distillation zone an over- In a modification of the process of the invenhead fraction is obtained containing a high contion a xylene cut, boiling between and centration of ethyl benzene, a residue contain- C.. is isolated and fed to a fractionating column ing a high concentration of meta-xylene and an of 50-100 plates, operating with a reflux ratio 50 intermediate cut containing substantial proporof at least 10 to 1, to remove overhead a fractions of para-xylene and meta-xylene. This intion boiling up to 138 C. and containing subtermediate cutiscooled to about --40 C. to -45 stantially all the ethyl benzene. The residue is C. and centrifuged, the solid obtained being then fed to a similar column operating with the essentially para-xylene. 'I'he'mother liquor from same reflux ratio to produce a concentrate with 65 this second centrifuging process is blended with 2 an approximate boiling range of 138 to 139.5 6., and containing about 40-50% of para-xylene, and a residue containing principally metaand ortho-xylenes. Although it is generally preferred the intermediate cut obtained in the first distillation stage and fed to a catalytic isomerisation stage in which part of the ortho-xylene and meta-xylene is converted into para-xylene and thereafter the para-xylene separated from the isomerisation product.

The method of separating the para-xylene formed in the isomerisation zone is not an essential feature of the present invention and may be effected by any process known in the art. If desired, the para-xylene may be separated by any of the methods described above for the separation of para-xylene in the feedstock, for example, by recycling the isomerisation product to the initial feedstock or to the feedstock to a second or later distillation stage.

The process conditions in the isomerisation stage will depend upon the nature of the catalyst used. It is known to carry out isomerisation reactions in liquid and vapour phase over aluminium halide catalysts supported or unsupported, and also over aluminium halide-hydrocarbon complexes. Using catalysts of this general char" acter usual process conditions include pressures in the range 1-50 atmospheres and temperatures in the range 38 C.-177 C.

A particularly advantageous method of operation of the catalytic isomerisation of the present invention using aluminium halide catalyst is described in the specification of British Patent No. 556,894.

Other catalysts which may be used for carrying out the catalytic isomerisation in accordance with the present invention are those synthetic catalysts which have a powerful catalytic effect on the cracking of high-molecular-weight hydrocarbons, such as the higher boiling cuts from petroleum, or from pure paraffin, from olefine, from naphthene hydrocarbons, or from aromatic hydrocarbons containing side chains of more than one carbon atom, into hydrocarbons of lower molecular weight. Typical examples of such catalysts are silica-alumina, silica alumina zirconia, silicamagnesia and silica-zirconia gels. The cracking catalysts of this type are usually operated at temperatures in the range 260 C.-677 C.

The isomerisation product is found to consist essentially of an equilibrium mixture of the three isomeric xylenes having a content of para-xylene of between 20 and 30%, and the equilibrium mixture may be fractionated into a para-xylene concentrate, and a fraction consisting essentially of orthoand meta-xylenes, the metaand orthoxylenes being returned to the isomerisation stage, while para-xylene is crystallised out from the concentrate, and if desired may be further purifled by re-crystallisation; the process of the invention has thus the considerable advantage that from a given quantity of xylene cut, it is possible to obtain an improved yield of para-xylene.

The xylene fraction may be derived from coal tar, the so-called 3 xylol out being a suitable fraction, or from petroleum fractions, such as straight run naphthas, and thermally and catalytically cracked naphthas. Aromatic hydrocarbons are removed from the naphtha fractions by an extractive process, such as solvent extraction, extractive distillation and azeotropic distillation. It may be possible to isolate an aromatic xylene fraction of sufficient purity from certain catalytlcally cracked naphtha fractions by distillation alone.

Such aromatic hydrocarbon fractions usually contain, in addition to ethyl benzene and the xylenes, small proportions of cyclo-parafilns and all) paraflins, and in the case of cracked naphthas, of unsaturated hydrocarbons. The aromatic hydrocarbon content of the fraction should be as high as possible in order to minimise carbon deposition on the catalyst in the isomerisation stage.

The invention is illustrated but in no way limited by the following example.

Example The feedstock to the process was the whole of the material of a aromatic extract of Iranian benzene boiling above 125 C. The feedstock was fractionally distilled in a plate column at a reflux ratio of 10 to 1 to yield three fractions as follows:

Fraction 1, (49.65 per cent of the whole) was the lowest boiling fraction and had the composition (wt. per cent).

Ethylbenzene 30.49 Para-xylene 28.13 Meta-xylene 38.99 Non-aromatics 2.39

Fraction 2, (16.74 per cent of the whole) had the composition Para-xylene 4.02 Meta-xylene 94.56 Ortho-xylene 1.42

Fraction 3, the residue (33.61 per cent of the whole) was ortho-xylene and higher aromatics, from which ortho-xylene was recovered by simple fractionation.

Fraction 1 was cooled to 45 C. and centrifuged. The solid so recovered (16.33% of the fraction and 8.12% of the original feed) was 93% para-xylene. The mother liquors (41.53% of the original feed) had the composition (wt.

Ethylbenzene 36.96 Para-xylene 16.31 Meta-xylene 43.79

Non-aromatic 3.45

These mother liquors were fractionated in a 100 plate column. From this fractionation were obtained three fractions as follows:

Fraction 4, which was 90% ethylbenzene concentrate containing 91.3% of the ethylbenzene originally present.

Fraction 5, the next boiling fraction (39.72% of the mother liquors and 16.50% of the original feed) of wt. composition.

Ethylbenzene 7.96 Para-xylene 35.13 Meta-xylene 55.81 Non-aromatics 1.10

Fraction 6, the residue, which was 23.4% of the mother liquors and 9.73% of the original feed, was 92.9% meta-xy1ene and 7.1% para-xylene.

Fraction 5 was cooled to 40 C. and centrifuged and the separated solid (23.35% of fraction 5 and 3.6% of the original feed) was 96.4% para-xylene and 3.6% meta-xylene. The mother liquors from this second centrifuging (12.65% of the original feed) had the wt.% composition Ethylbenzene 10.30 Para-xylene 16.46 Meta-xylene 71.92

Non-aromatics 1.32

This material was blended with fraction 2 and fraction 6 to yield a product of wt. composition Ethylbenzene 3.21 Para-xylene 8.66 Meta-xylene 87.10 Ortho-xylene 0.62 Non-aromatics 0.41

Ethylbenzene 4.0 Para-xylene 20.4 Meta-xylene 59.4 Ortho-xylene 16.0 Non-aromatics 0.6

This product was fractionated in the same way as the original feed to yield a para-xylene concentrate of composition close to that of fraction 5, a, meta-xylene concentrate of composition close to that of fraction 2 and an ortho-xylene residue. The recovery of para-xylene from the para-xylene concentrate followed the course described above.

We claim:

1. A process for the production of para-xylene from an aromatic hydrocarbon mixture boiling in the range 135 C. to 150 C. and derived from a. mineral oil selected from the group coal tar and petroleum, wherein the mixture is iractionally distilled to provide an overhead fraction rich in ethyl benzene, para-xylene and meta-xylene, an intermediate cut rich in meta-xylene and a residue fraction rich in ortho-xylene, the overhead fraction being passed to a low temperature-filtration zone wherein para-xylene is separated as a solid, fractionally distilling the residue obtained from said low temperature filtration to obtain a para-xylene enriched fraction containing at least 25% by weight of para-xylene and a meta-xylene enriched fraction containing less than 10% by weight of para-xylene and thereafter subjecting the para-xylene enriched fraction to low temperature filtration for the recovery of para-xylene and subjecting the said metaxylene enriched traction together with said intermediate cut rich in meta-xylene to catalytic isomerisation at a temperature within the range of 260 C. to 677 C. to convert at least part of said meta-xylene to para-xylene, and recovering para-xylene from the isomerisation product.

2. A process according to claim 1 in which ethyl benzene is recovered as an overhead fraction from the fractional distillation of said filtration residue.

3. A process according to claim 1 in which the residue obtained from the low temperature 111- tration of said para-xylene enriched fraction is subjected to catalytic isomerisation with said meta-xylene enriched fraction and said intermediate cut rich in meta-xylene.

4. A process according to claim 1 in which the low temperature filtration is conducted in the range C. to 40 C.

5. A process according to claim 1 in which the catalytic isomerisation is carried out in the presence of an aluminum halide as catalyst.

6, A process according to claim 1, in which the catalytic isomerisation is carried out in the presence, as catalyst, 01 a silica-containing gel selected from the group consisting of silicaalumina gel. a silica-alumina-zirconia gel, a silica-magnesia gel and a silica-zirconia gel.

STANLEY FRANCIS BIRCH. FREDERICK ARNOLD FIDLER. JOHN NORMAN HARESNAPE.

REFERENCES CITED The following references are of record. in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,940,065 Spannagel et a1. Dec. 19, 1933 2,282,231 Mam! May 5, 1942 ,403,757 Reeves July 9, 1946 2,435,792 McArdle et a1. Feb. 10, 1948 OTHER REFERENCES Norris et 81., The Rearrangement of the Xylenes by Aluminum Chloride." Jour. Amer. Chem. 800., vol. 61. pales 2131-2134 (Aug., 1939), 4 P 8 

1. A PROCESS FOR THE PRODUCTION OF PARA-XYLENE FROM AN AROMATIC HYDROCARBON MIXTURE BOILING IN THE RANGE 135*C. TO 150*C. AND DERIVED FROM A MINERAL OIL SELECTED FROM THE GROUP COAL TAR AND PETROLEUM, WHEREIN THE MIXTURE IS FRACTIONALLY DISTILLED TO PROVIDE AN OVERHEAD FRACTION RICH IN ETHYL BENZENE, PARA-XYLENE AND META-XYLENE, AN INTERMEDIATE CUT RICH IN META-XYLENE AND A RESIDUE FRACTION RICH IN ORTHO-XYLENE, THE OVERHEAD FRACTION BEING PASSED TO A LOW TEMPERATURE FILTRATION ZONE WHEREIN PARA-XYLENE IS SEPARATED AS A SOLID, FRACTIONALLY DISTILLING THE RESIDUE OBTAINED FROM SAID LOW TEMPERATURE FILTRATION TO OBTAIN A PARA-XYLENE ENRICHED FRACTION CONTAINING AT LEAST 25% BY WEIGHT OF PARA-XYLENE AND A META-XYLENE ENRICHED FRACTION CONTAINING LESS THAN 10% BY WEIGHT OF PARA-XYLENE AND THEREAFTER SUBJECTING THE PARA-XYLENE ENRICHED FRACTION TO LOW TEMPERATURE FILTRATION FOR THE RECOVERY OF PARA-XYLENE AND SUBJECTING THE SAID METAXYLENE ENRICHED FRACTION TOGETHER WITH SAID INTERMEDIATE CUT RICH IN META-XYLENE TO CATALYTIC ISOMERISATION AT A TEMPERATURE WITHIN THE RANGE OF 260*C. TO 677*C. TO CONVERT AT LEAST PART OF SAID META-XYLENE TO PARA-XYLENE, AND RECOVERING PARA-XYLENE FROM THE ISOMERISATION PRODUCT. 